1. Field of the Invention
The present invention relates to an absolute encoder.
2. Description of the Related Art
An encoder for measuring a position or angle is used for an industrial apparatus (processing apparatus) or measurement apparatus. For example, an optical encoder irradiates a scale with light from a light source, and detects transmitted, light or reflected light from the scale by photoelectric conversion devices. The light source and photoelectric conversion devices are attached to a moving or rotating measurement target object, and the scale is attached to a reference structure. Alternatively, the scale may be attached to the measurement target object, and the light source and the photoelectric conversion devices may be attached to the structure.
In general, encoders are roughly classified into incremental encoders and absolute encoders. The incremental encoders include incremental linear encoders for measuring a relative position and incremental rotary encoders for measuring a relative angle. The absolute encoders include absolute linear encoders for measuring an absolute position and absolute rotary encoders for measuring an absolute angle.
In an incremental encoder, transmission films or reflection films are formed on a scale at equal intervals, and light intensity signals output from photoelectric conversion devices are sinusoidal signals of a given period. A signal processing unit counts the number of waves of the sinusoidal signals, and divides a phase within one period into a number of sections, thereby improving the measurement resolution and accuracy. On the other hand, in an absolute encoder, a pseudo random number code is formed on a scale by transmission films or reflection films, and light intensity signals corresponding to the pseudo random number code are output from photoelectric conversion devices. A signal processing unit refers to a table representing the correspondence between a pseudo random number code and an absolute position or absolute angle, and calculates an absolute position or absolute angle corresponding to the light intensity signals output from the photoelectric conversion devices. In this case, similarly to the incremental encoder, it is possible to improve the measurement resolution and accuracy by dividing the phase of the pseudo random number code included in the light intensity signals into a number of sections.
As described above, an encoder includes a light source, a scale, photoelectric conversion devices, and a signal processing unit. However, light intensities (light intensity signals) output from the photoelectric conversion devices do not have a predetermined value for various reasons. For example, the light intensity of light emitted by the light source tends to change depending on a driving current value or temperature, and to lower with time. Variations occur in transmission films or reflection films formed on the scale in a manufacturing process, resulting in variations in transmittance or reflectance of the scale. Furthermore, since variations occur in transmittance or reflectance between scales, light intensities do not have a predetermined value. The light intensities also change depending on attachment of the scale or the light source and photoelectric conversion devices, and thus do not have the predetermined value.
To solve this problem, Japanese Patent Laid-Open Nos. 2001-311630 and 9-127141 respectively propose techniques of stabilizing light intensity signals output from the photoelectric conversion devices of an incremental encoder. Japanese Patent Laid-Open No. 2001-311630 discloses a technique of calculating the square root of the square sum of two light intensity signals having a phase difference of 90°, that is, an A- and B-phase signals, and controlling a light source so that the calculated value is constant. Japanese Patent Laid-Open No. 9-127141 discloses a technique of controlling the amplitude of the light intensity signals to be a predetermined value by extracting positive and negative peak values of the A- and B-phase signals, and adjusting the gain of the light intensity signals based on the result of comparing each peak value with a reference value.
On the other hand, Japanese Patent Laid-Open No. 2012-37392 proposes an absolute encoder in which a scale representing a pseudo random number sequence such as an M-sequence code has three transmittances or reflectances of 0%, 50%, and 100% rather than two values of 0% and 100%. This encoder sets a threshold between 50% and 100%, binarizes each light intensity signal into 0 or 1, and decodes the thus obtained code sequence (code), thereby measuring an absolute position or absolute angle. In this case, since the light intensity signals conform to a pseudo random number sequence, it is impossible to obtain sinusoidal signals having a constant amplitude, unlike a general incremental encoder. In other words, since regions having transmittances or reflectances of 50% and 100% are formed on the scale as pseudo random numbers, peak values at 50% and 100% randomly appear in obtained light intensity signals.
An encoder used for an industrial apparatus or measurement apparatus needs to measure a position or angle with high accuracy by stabilizing light intensity signals irrespective of a change in environment (temperature), a change with time, attachment of a scale or a light source and photoelectric conversion devices, manufacturing variations in scales, and the like.
As described above, however, unlike Japanese Patent Laid-Open Nos. 2001-311630 and 9-127141, in Japanese Patent Laid-Open No. 2012-37392, light intensity signals output from the photoelectric conversion devices are not sinusoidal signals with a constant amplitude but signals in which two amplitudes randomly appear. It is impossible to apply the techniques disclosed in Japanese Patent Laid-Open Nos. 2001-311630 and 9-127141 to adjust the light intensity signals to a predetermined value irrespective of a change in environment, a change with time, attachment, manufacturing variations, and the like.